In the inventor's U.S. Pat. No. 4,891,817, which has an assignee in common with the present patent application, there is described a multiple-prism, Littrow-mounted grating (MPL) dye laser oscillator. In addition to efficient narrow linewidth lasing (e.g., .DELTA..nu..ltoreq.1 GHz), this type of oscillator has the ability to yield a relatively low level of amplified spontaneous emission (ASE). A low ASE level is desirable for many applications including spectroscopy, isotope laser separation, and lidar and laser radar.
Amplified spontaneous emission in a dye laser has a number of basic characteristics which distinguish it from laser emission. First, most ASE occurs in the early stages of the emission process prior to narrow linewidth lasing. This stochastic ASE radiation is much lower in intensity than the main laser radiation and exhibits very much higher divergence. The ASE radiation is very broadband and hence all parasitic broadband reflections or unwanted modes in the resonator (laser oscillator) cavity should be minimized. A more comprehensive discussion of ASE radiation is given in an article entitled "Flashlamp pumped narrow-linewidth dispersive dye laser oscillators: very low amplified spontaneous emission levels and reduction of linewidth instabilities", by F.J. Duarte, et. al., Applied Optics, Vol. 29, No. 21, Jul. 20, 1990, pps. 3176 to 3179.
The free spectral range (FSR) of an optical cavity is defined as c/2L, where "c" is the velocity of light in free space (2.997925.times.10.sup.8 m./sec.) and "L" is the optical length of the cavity. For a cavity length L of 10 cm., the FSR is 1.5 GHz. As will be explained more fully hereinafter, it is desirable that the FSR be large (larger than the dispersive linewidth). This in turn means that the optical length of the cavity should be made short (for example, less than 10 cm).
For increased efficiency in isotope separation (for example, the separation of .sup.235 U from .sup.238 U) it is highly desirable that the linewidth of the laser emission be as narrow as possible, and tunable to an exact frequency. On the other hand, in medical treatment of internal tumors it is desirable to utilize a thin optical fiber in transmitting a laser beam to the point of application. This in turn means that the "quality" of the beam should be high in order for the beam to propagate efficiently through the optical fiber. And the "quality" of the beam is related to narrow linewidth.
It is desirable to have an optical cavity in a dye laser apparatus which is shorter than that with previous arrangements, is simpler and less expensive, and has lower ASE along with narrow-linewidth laser emission and good efficiency. In addition, it is desirable to have an increase in ruggedness of the physical apparatus and greater temperature stability compared with prior arrangements.